The preparation of organopolysiloxanes from short-chain organo(poly)siloxanes is described in detail in W. Noll, Chemie und Technologie der Silicone (Chemistry and Technology of the Silicones), 2nd Edition 1968, Chapter 5.4. Even though the organopolysiloxanes are colorless, clear, odorless and inert, they may contain impurities which cause changes in color, in particular during storage under the influence of heat, subsequent clouding, inhibition of catalysts, for example during addition reactions with platinum, a lack of heat stability, a lack of dielectric strength or formation of odors, especially after exposure to light. Impurities occur not only when homogeneous catalysts, such as PNC1.sub.2, are used, but are also present when heterogeneous catalysts, such as acid bleaching earth, are used.
In the conventional processes for preparing organopolysiloxanes, they contain impurities in extremely small amounts, however, these impurities have not been identified analytically.
The effects of the impurities can be eliminated in part by an after-treatment with an adsorbent, such as active charcoal. However, the adsorbents retain large amounts of the polyorganosiloxane, thereby resulting in a significant loss in yield. To date, it has not been possible to purify organopolysiloxanes contaminated with impurities which inhibit the addition reaction with platinum catalysts.
The preparation of odor-free silicone oils by equilibration of organopolysiloxanes in the presence of an acid catalyst and an added metal is described in DD-A-240 897 (H. Kronig et al., published on Nov. 19, 1986 for VEB Chemiewerk Nunchritz, GDR). The unpleasant smell associated with the silicone oil is said to be eliminated by the added metal. However, if a heterogeneous catalyst is used, a mixture of catalyst and added metal is obtained which can scarcely be reprocessed and can present disposal problems. The amount of metal required is therefore high and the catalyst can only be reused after an expensive treatment, if at all. Homogeneous acid catalysts react with the added metal and are not suitable for this process.
Therefore, it is an object of the present invention to provide a process for purifying organopolysiloxanes. Another object of the present invention is to provide a process for purifying organopolysiloxanes regardless of their process of preparation. A further object of the present invention is to provide a process for preparing organopolysiloxanes in which a heterogeneous catalyst can be used several times and in which no disposal problems arise.